(1) Field of the Invention
The present invention relates to an improved method of producing oxygen concentration cells, and more particularly relates to a novel method of forming an electrode of oxygen concentration cell.
(2) Description of the Prior Art
It has been known that an oxygen concentration cell, which operates by utilizing the oxygen ion-conductivity of solid electrolyte, is used as a sensor for detecting the presence or absence of oxygen. When the oxygen concentration cell is used as the above described sensor, the electrode is required not to be peeled off even when the electrode is exposed for a long period of time to a high temperature gas to be detected. In order to satisfy this demand, there have hitherto been proposed various methods, for example, (a) a method wherein a sintered oxygen ion-conductive solid electrolyte matrix is subjected to acid etching, sandblasting or the like to form a rough surface; (b) a method wherein a porous film consisting of the same material as that of a detecting element is formed on the surface of the detecting element (Japanese Patent Laid Open Application No. 12,392/78); (c) a method wherein a porous film is formed on the surface of a sintered detecting element (Japanese Patent Laid Open Application No. 29,187/78); (d) a method wherein a substance consisting of the same material as that of a detecting element is flame-sprayed by means of a plasma spray on the surface of a sintered detecting element to form a porous film thereon (Japanese Patent Laid Open Application No. 78,885/78); and the like.
However, these methods have the following drawbacks, and are still insufficient for practical use. That is, in the method (a), the mechanical and thermal strength of the matrix are noticeably deteriorated, and the matrix cracks often during the use. In the method (b), relatively fine particles (not larger than 20 .mu.m) after pulverizing step are directly adhered to the matrix to form a porous film on the matrix surface, and therefore the electrode is apt to peel together with the porous film during the use. The reason is probably as follows. Since the electrode is penetrated into the voids of the porous film, the electrode has a somewhat long duration time, but when the electrode is exposed to a reducing atmosphere, the electrode is sintered to cause growth and change of properties of particles and to cause a high mechanical stress in the porous portion of the film, and moreover the porous film is very weak against mechanical stress due to its porous structure, and hence the porous film cracks, and peels together with the electrode. In the method (c), a slurry is applied to the surface of a previously sintered detecting element and the assembly is sintered to form a porous film. Therefore, the porous film is not firmly bonded to the matrix, and the porous film is apt to peel easily during the use. In the method (d), the reaction between the sprayed substance and the sintered detecting element is weak, and the sprayed substance peels together with the electrode. In order to prevent this drawback, when the plasma spraying is carried out at a high temperature, a thermal gradient is caused in the detecting element matrix, and the matrix is apt to crack easily. While, when the plasma spraying is carried out at room temperature, and then the resulting assembly is sintered, the thermal gradient does not occur in the matrix, but the procedure is troublesome.